Abrasive jet or waterjet systems have a cutting head that produces a high-velocity fluid jet or waterjet that can be used to cut or pierce workpieces composed of a wide variety of materials. Abrasives can be added to the waterjet to improve the cutting or piercing power of the waterjet. Adding abrasives results in an abrasive-laden waterjet referred to as an “abrasive waterjet” or an “abrasive jet.” Abrasives are generally drawn into the abrasive water jet by air flow resulting from a low pressure (vacuum) generated by the Venturi effect of pressurized water flowing through the abrasive cutting head. Abrasives are typically metered to the open end of a conduit, such as a tube, coupled to the abrasive water jet cutting head and “vacuumed” into a mixing chamber to be combined with the high pressure fluid and expelled through a mixing tube or nozzle and directed against a workpiece.
Certain materials, such as composite materials and brittle materials, may be difficult to pierce with an abrasive jet. An abrasive jet directed at a workpiece composed of such material strikes a surface of the workpiece and begins forming a cavity. As the cavity forms, a hydrostatic pressure may build within the cavity. This hydrostatic pressure may act upon sidewalls of the cavity and negatively impact the workpiece material. In the case of composite materials such as laminates, such hydrostatic pressure may cause composite layers to separate or delaminate from one another as the hydrostatic pressure exceeds the tensile strength of the weakest component of the materials, which is typically the composite binder. In the case of brittle materials such as glass, polymers, and ceramics, the hydrostatic pressure may cause the material to crack or fracture. Other aspects or effects of the abrasive jet other than the hydrostatic pressure may, in addition or as an alternative to the hydrostatic pressure, cause or result in damage to the material during abrasive jet piercing operations.
Conventional techniques for mitigating piercing damage to materials include low pressure piercing, pressure ramping and vacuum assist devices. Low pressure piercing generally involves operating the abrasive water jet cutting system at a lower pressure for piercing than cutting. Once piercing is completed, pressure increases and cutting commences. Pressure ramping can involve using a reduced water pressure to form the waterjet and ensuring that abrasives are fully entrained in the waterjet before the hydrostatic pressure reaches a magnitude capable of causing damage to the material being pierced. A vacuum assist device can be used to draw abrasive into a mixing chamber of a waterjet cutting head prior to the arrival of water into the mixing chamber. Such a technique can prevent a water-only jet from striking the surface of the material.